Quantitatively determining the metal content of solutions of rhodium



Patented June 29, 1937 PATENT @FFICE 2,085,177 QUANTITATIVELY DETERRHNING THE MET- AL CONTENT 0F S DIUM Herbert E. Zschiegner, Ocean GLUTIGNS 0F RHO- Grove, N. 3., as-

signor to Baker & Company, llnc., Newark, N. J., a corporation of New Jersey N 0 Drawing.

Application February Serial No. 63,597

4 Claims.

This invention relates particularly to a method of ascertaining the amount of metal in substantially pure solutions of rhodium.

Heretofore gravimetrical analysis has been the only dependable method of determining the quantitative platinum metal content of platinum metal solutions. Such a procedure is expensive in that it calls for the special skill and knowledge of a trained chemist, and considerable time is required for effecting the analysis.

Rhodium is frequently used in electro-plating baths where it is deposited by electrolysis on metal or other conducting surfaces for tarnishproofing and other purposes.

In electroplating these metals from a solution it is of great importance to the plater to know the quantitative metal content of the bath, and it is highly desirable in many cases to effect an analysis in a short time. Usually the plater starts out with a bath having a definite metal concentration, but during use the metal concentration decreases. The plater, however, has no adequate method available to ascertain how much metal has been abstracted from the bath or how much metal has been left in the bath. As a result, the quality of the deposit will tend to vary in thickness and appearance unless the plater prolongs the time of plating or replenishes the bath. The electroplating method isv therefore often predicated on the individual platers judgment and accordingly produces unsatisfactory results.

To avoid this, the platers now frequently submit samples of their baths to an expert analyst for determination of the quantitative content of the platinum metal in the respective baths. Such a procedure requires usually a whole week and necessitates interruptions of operations for extended periods of time. After the analysis has been made, the plater can replenish the bath with metal salts or concentrated metal solutions, if necessary, to bring the metal concentration up to the standard concentration.

A prime object of the invention is to provide a simple and extremely practical method of determining the metal concentration of rhodium solutions, which can be carried out by anyone whether skilled or not, and which can be accomplished in a small fraction of the time necessary for the analyses known at the present time. Such a method is extremely economical since it permits the operator to make his own determinations and does away with the necessity of sending samples to a skilled and experienced analyst. This, of course, is of the greata known quantity of rhodium, with a bromide compound which as such will form a colorless solution in water and not precipitate the metal, and an acid, so as to develop in the solution a color the intensity of which is dependent on the amount of the metal present; then treating in the same manner a like quantity of a rhodium solution to be tested and containing an unknown quantity of the same rhodium; and thereafter comparing the colors of the two solutions.

In order to bring out the most intense color of any one solution it is desirable to add an excess of the bromide and then heat until the color is developed, which usually takes place when the solution is heated to a gentle boil.

Preferably the solution containing the known quantity of rhodium then will be diluted with a known quantity of water, and the solution being tested will then be gradually diluted with water until its color matches the color of the known or standard solution. Then inasmuch as the quantitative content of metal of the standard solution is known, the quantitative content of the metal in the solution being tested can be readily ascertained by comparing the quantities of the two solutions.

To more fully explain the invention, the following specific examples of the method may be considered.

EzrampZe 1 A graduated test tube containing two (2) grams sodium bromide and two (2) cubic centimeters of a rhodium solution containing dilute sulphuric acid and two (2) grams of rhodium (in the form of phosphate) per liter, was heated to a gentle boil over a gas flame for one (1) minute. Some elementary bromine was expelled and apparently a rhodium-bromide compound was formed. The color of the solution in the test tube turned to deep rose. This may be considered the known or standard solution. Another similar test tube containing two (2) grams of sodium bromide and two (2) cc. of a similar rhodium solution having an unknown quantity of rhodium, was then heated to a gentle boil over a gas flame for one (1) minute. The color of the solution turned to rose. This represents the unknown or test solution. After heating. both tubes were permitted to cool. The test tube containing the solution having the known quantity of rhodium was then diluted with water to the twenty (20) cc. mark on the tube. Thereafter the tube containing the test solution having the unknown quantity of rhodium was diluted in steps until the color of its solution matched the color of the diluted known or standard rhodium solution. Since the quantitative rhodium content of the first solution is known, it is easy to calculate the quantitative rhodium content of the test solution by comparing the quantity of solution present in the two tubes.

The following examples are intended simply as illustrative of the manner of developing color in different platinum metal solutions, and the solutions may be either standard or test solutions for comparison with other similar solutions as above described.

Example 2 developed was identical with the color obtained in Example 1.

Example 3 Two (2) cc. of a rhodium solution containing rhodium sulphate in dilute sulphuric acid were treated with two (2) grams of potassium bromide. A rose color was developed.

Example 4 Two (2) cc. of a rhodium solution containing rhodium phosphate in dilute sulphuric acid were treated with two (2) grams of ammonium bro-- mide. A rose color was developed.

It will be obvious that the foregoing examples are given as purely illustrative examples only. Any solution, the rhodium content of which is known, may be used as the standard solution for comparison. The amount of sodium bromide may also be varied widely. The temperature to which the solutions are submitted may also be varied, provided that both solutions are heated at approximately the same temperature for approximately the same period of time. It will be obvious also that the method of comparison may be varied within wide limits. substitutes for the standard solution which simulate the color of the standard solution after heating. This may be done by a color chart or colored cellophane, colored glass or colored solutions. In

such a case it will be necessary merely to prepare the test solution and then compare its color with the color of the standard substitute. It is pos sible to simplify the determination by graduating test tubes in divisions, each of which is equal to one-tenth of a gram of a platinum metal per liter.

As indicated before, any bromide may be used which forms a colorless solution in water or dilute acids. Such bromides are for instance sodium bromide, potassium bromide, and ammonium bromide. It should be noted, however, that the bromide used must not form a precipitate with the rhodium present.

Any solution which does not react directly with It is possible to use a bromide must first be heated with an acid which does react with a bromide such as, for instance. sulphuric acid, and then treated as described.

As the difierent shades of color are easy to match, accurate determinations of the quantitative platinum metal contents of a solution of said metal can be obtained within a limit of error of substantially less than 10%. For practical purposes such a determination is entirely satisfactory, and constitutes an important advance in the art.

The relative accuracy of my method and the known gravimetrical analyses is shown by the following comparison of tests which have been made.

Rhodium deter} gfg g ggggf' mmeid by method of m metrical analyses invention y Grams per liter Grams per liter 1. 56 1. 60 1. 71 1. 85 1. 1O 1. 10 l. 15 1. 10 2. 4 2. 58 2. 02 2. 00 2. 18 2. 1O 74 1. 38 1. 39

Having thus described my invention, what I claim is: Y 1

l. The steps in the method of determining the quantitative rhodium metal content of substantially pure rhodium solutions, consisting in preparing a solution containing and unknown quantity of rhodium, a bromide compound which does not precipitate the rhodium and an acid which reacts with the bromide and the rhodium to form a rhodium-bromine compound, and comparing the color of said solution with a standard color representing a known quantitative content same metal in a similar solution.

2-. The step in the determination of the rhodium metal content of substantially pure rhodium solutions, consisting in preparing a solution containing an unknown quantity of rhodium, a sodium bromide and an acid which reacts with the sodium bromide and the rhodium to form a rhodium bromine compound, and comparing the color of said solution with a standard color representing a known quantitative content of the same metal in a similar solution.

3. The steps in the method of determining the rhodium metal content of substantially pure rhodium solutions, consisting in preparing a solution containing a known quantity of rhodium, a bromide compound which does not precipitate the rhodium and an acid which reacts with the bromide and the rhodium to form a rhodium metal-bromine compound, for comparing the color of said solution with the color of another similar solution to be tested and containing an 1 form a rhodium bromine compound, for comparing the color of said solution with the color of another similar solution to be tested and containing an unknown quantitative content of the same metal.

HERBERT E. ZSCI-IIEGNER. 

